1. Field of the Invention
The present invention relates to an image formation method and apparatus for use in an apparatus such as a copying machine, a facsimile machine, or a printer, and more particularly to an image formation method and apparatus capable of producing high quality images by use of a wet-type liquid toner developer.
2. Discussion of Background
Recently, in the field of electrophotographic technology, a digital-type latent image formation method using laser beams has been developed. It is possible to write dot images with a resolution of well over 800 DPI by this method. The development of an image formation method and apparatus which can provide images with high resolution and high quality by using the above-mentioned latent image formation technology is greatly required.
It is necessary to reproduce highly accurate dot images in order to obtain high quality printed images. In such a case, the scattering and collapse of toner particles which occur in the course of the transfer of toner images to a transfer sheet and the fixing of toner images to the transfer sheet with the application of heat or pressure are the main problems to be solved, especially for color printing.
When the particle size of toner particles is minimized, toner images with high resolution can be obtained, and color reproduction can be improved in the case where color printed images are to be obtained by superimposing images with different colors, because the thickness of the toner images transferred to a transfer sheet as a final recording medium can be decreased. Furthermore, the curling of the transfer sheet and cracking of the toner images can be prevented when the particle size of the toner particles is reduced and accordingly the thickness of the transferred toner images is decreased.
There are two types of toners: a powder toner for use in a dry-type development, which is employed together with a powder carrier by triboelectric charging (hereinafter referred to as dry-type toner); and a toner for use in a wet-type development, which is employed by dispersing the toner in a solvent used as a liquid carrier (hereinafter referred to as wet-type or liquid toner development).
When the above-mentioned dry-type toner consists of finely-divided particles with a particle size of the order of 5 .mu.m to submicrons, there are problems that it is difficult to prevent the toner from scattering and to remove any residual toner from the photoconductor which is employed as a latent-image bearing member, after the transfer process.
On the other hand, the same problems as mentioned above do not occur when the wet-type toner is dispersed in a solvent and employed for development. Therefore, the wet-type toner with a small particle size can be easily used.
Furthermore, the toner with a lower melting point is more advantageous with regard to power consumption and high-speed printing. Although the dry-type toner having a low melting point has a blocking problem, the wet-type toner which is dispersed in a solvent during use does not have such a problem. Therefore, the wet-type toner with a low melting point (a low softening point) can be used for high-speed printing.
However, in the case where an electrostatic image transfer method is employed, in which a transfer sheet is brought into contact with the surface of a photoconductor on which a toner image is formed by developing a latent electrostatic image with a wet-type developer, an electric field for image transfer is formed by applying an electric charge to the back side of the transfer sheet with a corona discharger, and the toner image on the photoconductor is transferred to the transfer sheet, the charged transfer sheet is electrostatically attracted by the photoconductor. As a result, the toner image is crushed, so that it becomes difficult to faithfully reproduce line images with accurate line widths and dots with accurate halftone dot areas. For example, even when a toner with a particle size in the order of submicrons is employed for development, the resolution of the obtained images is at most 10 lines/mm.
Moreover, a solvent must be provided between the surface of the photoconductor and the surface of the transfer sheet in order to induce electrophoresis of the toner under the above-mentioned electric field for image transfer. As a result, the solvent wets the transfer sheet. The result is that it becomes difficult to peel the transfer sheet away from the photoconductor, because the surface tension of the solvent and the electrostatic adsorbability of the transfer sheet to the photoconductor work together. Furthermore, it is necessary to sufficiently dry the wet transfer sheet before discharging the transfer sheet from an outlet of a copying machine.
In addition to the above-mentioned method, for example, Japanese Laid-Open Patent Application 63-34573 discloses an image transfer method in which a wet-type toner image is formed on a photoconductor and electrostatically transferred to an intermediate transfer medium. In this image transfer method, in order to reduce the amount of solvent which adheres to a transfer sheet when the toner image is transferred to the transfer sheet, a wet-type toner image formed on a photoconductor is electrostatically transferred to an intermediate transfer medium, and the solvent which adheres to the intermediate transfer medium together with the toner is evaporated by the application of heat thereto. The transfer sheet is then brought into pressure contact with the intermediate transfer medium which bears the toner image thereon under the application of heat in order to carry out the image transfer and image fixing at the same time.
In this method, the transfer of the toner image is carried out twice: the toner image formed on the photoconductor is transferred to the intermediate transfer medium, and then the toner image formed on the intermediate transfer medium is transferred to the transfer sheet. As a result, the resolution of the image finally formed on the transfer sheet is lowered. Even though the latent electrostatic image is formed on the photoconductor by producing a dot image with a resolution much greater than 800 DPI, the toner image obtained on the transfer sheet has a resolution of only about 800 DPI.
In addition to the two above-mentioned methods, the following references disclose image formation methods and apparatus which appear to be related to the present invention. These image formation methods and apparatus, however, use a dry-type developer. These references neither suggest nor teach about a method of obtaining images with high resolution and high image quality using a wet-type developer.
(1) Japanese Patent Publication 64-1028 discloses an image formation method for obtaining toner images with high resolution, in which a toner image is formed on a photoconductor by dry-type development and transferred to an intermediate transfer member with the application of pressure, and then the transferred toner image is fused and transferred to a transfer sheet by the application of pressure and heat thereto using a heat roller. This image formation method is proposed as a method which can replace an electrostatic image transfer method using an electrode for image transfer such as a corona discharger, which has the problem that the resolution of a dry-type toner image is lowered because of the turbulence of the electric field which occurs in the course of the transfer of the dry-type toner image formed on the photoconductor to a transfer sheet.
(2) Japanese Laid-Open Patent Application 60-22171 discloses an image formation method in which a latent electrostatic image is formed on a photoconductor and electrostatically transferred onto an electric-charge bearing member, the latent electrostatic image thus transferred to the electric-charge bearing member is developed with a dry-type developer to form a toner image, and the toner image thus formed on the electric-charge bearing member is transferred and fixed onto a transfer sheet at the same time with the application of pressure and heat.
(3) Japanese Laid-Open Patent Application 58-107564 discloses an image formation method in which a latent electrostatic image is formed on a photoconductor and electrostatically transferred onto an electric-charge bearing member, the latent electrostatic image thus transferred to the electric-charge bearing member is developed with a dry-type developer to form a toner image, the thus formed toner image is transferred to an intermediate member with the application of pressure, and then the toner image thus formed on the intermediate member is transferred and fixed onto a transfer sheet at the same time with the application of pressure and heat.